I've played mp3's on my 030 50mhz, but of course reduced quality and mono sound is needed in order to make it work. As a result, sound is quite bad.
030 is too weak for playing mp3's, it is useles but for testing purposes or as a curiosity.
In fact to do what a 030 does playing mp3's you'll need a i486 on a PC. If you want to play music with fairly good sound quality you'll need at least a 060. In fact dedicated processors on MP3 players, have a higher clock rate, and they're specifically designed for that code, so an "all round" CPU at 50mhz is not a big deal.